There are currently a number of mobile telecommunications technologies available to the general user, offering a variety of services, in particular voice and data services, using a mixture of circuit-switched and packet-switched protocols. In general the more established systems have a wider coverage than newer systems which are still being introduced. Typically the newer technology is initially installed in a limited range of small cells where there is a particular need for coverage, with a partner macrocell network providing circuit-switched Fall Back (CSFB) services. In such an arrangement, in locations where both systems are available, network services, such as voice, are routed over a circuit switched network and data over a packet-switched network.
An added complication is that not all mobile user terminals are fully compatible with all mobile technologies. A particular measure which is used in the Long Term Evolution (LTE) standard is known as “Circuit Switched Fall Back”, which is a mechanism that allows user terminals that do not have a capability to operate Voice-over-LTE (VoLTE) technology, and therefore cannot place voice calls over the packet-switched LTE (or “4G”) system, to “fall-back” to the older circuit-switched 2G or 3G (or “UMTS”) systems to make or receive voice calls whilst remaining attached to a 4G/LTE packet-switched radio access network (radio access network) for data services.
SMS text messages can be delivered over the LTE radio access network, if the SGs Interface defined in 3GPP TS 29.118 is supported, whereas non-VoLTE capable devices cannot receive voice calls over the LTE radio access network, which is why the Circuit Switched Fallback mechanism is required.
Traditionally, voice and text messages have followed the same route through the network since they had to be delivered over the same access network, using the same international mobile subscriber identity (IMSI) and mobile switching center server (MSCS), there being no alternative radio delivery mechanism for text messaging. However, the 4G system provides a way to deliver text messages in a different way to the delivery method for voice, and for which there is no need for the device to fall-back to the 2/3G network, and therefore there is no need to route the SMS via the Circuit Switched core if it is being delivered via the 4G RAN.
Current Circuit Switch Fall Back procedures in 3GPP require a dual attachment when users attach to a 4G network, one attachment against the 4G Evolved Packet Core (EPC), and a separate IMSI attachment to a MSC/VLR allocated in the Circuit Switched (circuit-switched) voice core for the purpose of routing voice calls and text messages to the device. When the 2/3G radio access network (and associated circuit-switched core) and the 4G radio access network (and associated EPC core) belong to different operators, the allocation of an MSC server in the 2/3G partner operator's voice core network also implies that SMS terminating messages may have to be routed via the partner's circuit-switched core network, even when the subscriber is attached to its home operator's 4G radio access network. This is suboptimal since SMS messages would be routed via the 2/3G voice core before final delivery to the UE via the 4G access, resulting in an unnecessary dependency on the 2/3G voice core. There may also be commercial implications when the 4G and 2/3G operators are not the same.